I have your same problem but have no option to enlarge the vent duct due to the size of the space between the wall and ceiling studs. My previous vent hood was a 36" Cavaliere 900 cfm for a 36" Dacor Epicure cooktop. My new range is 36" DCS with 6 burners. Because I wanted a more substantial look, and after much searching and debating, I bought an AirPro 860 cfm (link below) The Cavaliere hood was more than what I needed for the 36" Dacor. I never turned it up past 1 or 2 while cooking. On 6, the highest speed, it was kinda loud but since I never used it on high it wasn't a problem. The AirPro has not been installed yet so I don't know what the actual noise level will be but it's rated at 35 / 0.7 to 67 / 6.5 (Lowest to Highest Speed). I just couldn't see paying thousands of dollars for a vent hood especially since my $400 Cavaliere had worked fine for 5 years. During my hood research one of the retailers did tell me there was a vent outlet adapter available online but because our contractor wasn't thrilled with that idea, I stayed with the 6" hood duct. Hope this helps. Good luck in your search for the right hood. Here is a link that might be useful: AirPro Hood

opaone, I put in a 7" duct like you suggest so the CFM is not decreased " Reducing duct size with these fans will result in decreased velocity, not increased. " Reducing the duct area does reduce the CFM because of increased restriction, that is true. It increases velocity at the expense of reduced CFM . But, it's a weak blower. I decided not to choke it further. stkn, I considered an in-line fan and other options, knowing an OTR micro is not the best extractor. Decided against it for various reasons. It's installed, it works, roof does not leak (important!), pulls most of the frying smell out.

Rectangular ducts have effective diameters that are relatively low compared to the sectional area equivalent. Stick to round where possible. I would follow all the steps I've outlined here many times for determining the hood requirements, then find a hood and blower combination that meets the requirements, and then have a transition made (if not available). If the hood came with an 8-inch duct interface, and an adapter for 6-inch, I would use the 8-inch opening and transition to seven inches. But first, the hood damper needs to be selected that mates to the hood. In the example, an 8-inch damper will likely have lower pressure loss. Did you mean an actual 600 CFM from a 900 CFM rated blower, or an actual 400 CFM from a 600 CFM rated blower? In general, in the full power mode at the flow rate actually achievable, velocity in the duct should be in the 1000 to 2000 ft/min range, so you can calculate velocity for various duct diameters and blower CFM.

Regardless if you can or you can't, you should ask your building inspector.